In an electron beam irradiator, an electron beam is generated and accelerated in a high-degree vacuum and is emitted into the atmosphere to irradiate an object with electrons and to cause a chemical reaction therein to change the chemical properties thereof. Although the electron beam irradiator is used for various purposes, it is most often used for polymerization in applications to cross-link an electric insulator that coats an electric wire, a heat-shrinkable tube, formed polyethylene, a rubber tire and so forth. The electron beam irradiator can be used to sterilize medical equipment, process foodstuffs and feedstuffs, denitrate and desulfurize smoke, and harden a liquid resin for coating, printing, lamination, magnetic medium processing, and so forth. The amount of energy of the electron beam is expressed by the acceleration voltage, which is commonly about 100 kV to 10,000 kV, and differs depending on the purpose of the irradiation by the electron beam. The amount of energy of the electron beam is sometimes classified into a low range for 300 kV or less and a medium and high range of more than 300 kV. Since an electron beam in the low range of energy only reaches the surface of the object and the vicinity thereof, the beam is used for surface processing. For example, an electron beam in the low range is used to harden a liquid resin for coating, printing, lamination, magnetic medium processing, integrated circuit board processing, and so forth. Therefore, the electron beam irradiator is not a measuring apparatus.
A large number of measuring devices employing electron beams have already been provided. The measuring devices include an electron microscope, a reflection-type high-energy electron beam diffraction device, a low-energy electron beam diffraction device, and so forth. In each of the measuring devices, an object is put in a high-degree vacuum and the device is required to measure the distribution of angles of scattered electrons from the object and the angles of diffraction electrons therein.
The electron beam irradiator is a processing apparatus that irradiates electrons upon the object to cause a chemical change therein to alter the quality thereof. Therefore, the electron beam irradiator is different from a measuring device that uses the electron beams and measures the intensity distributions of scattered electrons, secondary electrons, diffracted electrons, and so forth.
An electron beam irradiator typically comprises a high DC voltage power supply, an electron gun, an accelerating tube, a scanning horn, an irradiation window, an object conveyor, and a vacuum degassing unit. The high DC voltage power supply is for generating a high voltage necessary to accelerate the electrons, and is made of a Cockcroft-Walton circuit, Delon-Grainahel circuit, Dinamitron DC power supply, or the like. If the current from the high DC power supply is as weak as 1 micro-amp to 1 milli-amp, a van de Graff type supply may be used.
In the electron gun, electricity is applied to a filament in a vacuum to emit thermoelectrons and attract the thermoelectrons toward an anode to separate the thermoelectrons. In the accelerating tube, annular electrodes are juxtaposed and negative voltages are distributed thereto in the direction of the flow of the electrons to vertically accelerate them downward. In the scanning horn, the electrons vertically proceeding downward are subjected to magnetic fields in two directions to cause the electrons to perform scanning motions in two directions.
If the energy of the electron beam in the electron beam irradiator is in the low range, the irradiator may not have the scanning horn. Since an electron beam of very high speed is curved by the scanning horn, the horn needs to have a long proceeding distance for the electron beam. For that reason, if the electron beam irradiator has a scanning horn, the irradiator will be bulky. Since it is difficult to use practically a non- scanning-type electron beam irradiator whose electron beam is in the medium and high range of energy, an electron beam irradiator having an electron beam is in the medium and high range of energy is usually of the scanning type. FIG. 3 shows a schematic view of an electron beam irradiator with a scanning horn.
Since an electron beam irradiator whose electron beam is in the low range of energy is required to be compact, the irradiator is not provided with a scanning horn and is of the non-scanning-type, which is sometimes also called the area type. In such a low energy irradiator, the length of the accelerating tube can also be made small, and the electron beam can be accelerated in some cases by using only a pair of electrodes. Therefore, the accelerating tube. can be made compact.
The interior opening of each of the electron gun, accelerating tube, and scanning horn (which is not provided in some electron beam irradiators) of the electron beam irradiator shown in FIG. 3 are all subject to a high-degree vacuum. A vacuum degassing unit degases the interior opening of each of them to the high-degree vacuum. The irradiation window forms a border between the vacuum and the atmosphere. The interior opening of each of the accelerating tube and the scanning horn is in high-degree vacuum, while the object is placed in the atmosphere. Therefore, the accelerating tube and the scanning horn constitute a vacuum container.
If the scanning horn is provided in the electron beam irradiator, the bottom of the scanning horn has the irradiation window. If the scanning horn is not provided in the electron beam irradiator, the bottom of the accelerator would have the irradiation window. In either case, the irradiation window is formed in the electron beam opening from the vacuum container.
The irradiation window is made of a material that blocks air to maintain the high-degree vacuum but allows the electron beam to pass. Since the electron beam comprises radiation of low penetration power, the thickness of the material must be very small. For that reason, a titanium foil of about 15 to 30 microns in thickness or an aluminum foil of about 30 to 70 microns in thickness is used as the material. The difference between the pressure on the inside of the material and that on the outside is nearly equal to atmospheric pressure because the interior opening of the vacuum container constituted by the accelerating tube and the scanning horn is in the high-degree vacuum and the object is in the atmosphere.
If the irradiation window is of small thickness it will be be deformed into the high-degree vacuum and will be subject to a high degree of tension if the area of the irradiation window is large. The thickness of the material should be made large in order to enable the material to withstand strong tension. If the thickness of the material is large, however, much of the electron beam will be absorbed and a large energy loss will occur. Even if the thickness of the material is small, the electron beam loses some of its energy because each of the electrons is a charged particle of small mass. A thin and durable titanium foil is often used as the material for covering the irradiation window.
An object conveyor carries an object from an inlet port to a position directly beneath the irradiation window, and thereafter carries the processed object to an outlet port. A conveyance mechanism is provided in the base of the conveyor through which X-rays cannot pass. Inlet and outlet port preparation chambers, which are closed by shutters, are provided at the ends of the conveyor. Since X-rays are emitted when the electron beam collides against a substance, it is necessary to block the X-rays.
A conventional device for holding the foil in a scanning type irradiator is described with reference to FIGS. 5 and 6. The irradiation window is provided in the lower portion of a scanning horn 1, which constitutes a part of a vacuum container. The longitudinal section of the scanning horn 1 is shaped as an isosceles trapezoid so that the scanning horn diverges toward the bottom thereof and the front and rear walls of the scanning horn extend in parallel with each other. The distance between the front and rear walls of the scanning horn 1 is small. The side walls of the scanning horn 1 are oblique. In a space defined by the front wall, rear wall, and oblique side walls of the scanning horn 1, an electron beam is scanned by alternating magnetic fields oriented in the longitudinal direction of the cross section of the scanning horn and the direction perpendicular to that longitudinal direction.
A foil 3 is supported at the bottom of the scanning horn 1. The foil 3 is preferably oblong and has its four edges pinched between the top of a foil holder 2 and the bottom of a flange 11 provided on the lower portion of the scanning horn 1. The flange 11 and the foil holder 2 have a large number of bolt holes provided along the periphery of each of the flange and the foil holder and corresponding to each other. Bolts 8 are inserted into the bolt holes and tightened by nuts (not shown in the drawings) or tapped holes provided in the foil holder 2 as shown in the drawings, so that the foil holder 2 and the foil 3 are secured to the flange 11. The top of the foil holder 2 and the bottom of the flange 11 must be parallel so that there is no gap between the foil 3 and each of the flange 11 and the foil holder 2, because it is necessary to subject the inside of the foil to a high-degree vacuum and the outside thereof to the atmosphere to maintain the high-degree vacuum in the scanning horn 1 by the foil.
In the conventional device for holding the foil for the irradiation window it has been particularly troublesome to replace the foil. Since a pressure difference nearly equal to 1 atmosphere acts against the thin foil 3, the foil is subject to a tension force equal to the product of the pressure difference and the area of the foil. When the electron beam passes through the foil 3, much if the energy of the beam is absorbed to generate heat that raises the temperature of the foil. Although cooling air is blown against the bottom (exterior) surface of the foil 3 to cool it, the beam passage area thereof is heated to a high temperature. In other words, the foil 3 is subject to high tension as a result of the pressure differential and also to heat, which fatigues the foil. For that reason, the foil 3 must be replaced every several months. If during te process the foil 3 is broken, the accelerating tube of the electron beam irradiator will no longer be subject to high vacuum and the electrodes of the accelerating tube are likely to be damaged. Therefore, it is necessary to replace the foil 3 without breaking it.
In order to replace the foil 3, a person must enter the irradiation chamber, remove the many bolts 8 (the number of which may be as high as a hundred), remove the old foil and attach a new one to the foil holder 2 by a tape. At that time, it is necessary to attach the new foil under tension to keep the new foil tight. This is difficult work. The foil holder 2 fitted with the new foil is lifted to the flange 11 and coupled thereto by the bolts 8. The foil holder 2 is so heavy that it is very hard for only one person to lift it. Therefore, at least two persons are needed to lift the foil holder 2. It is then necessary to tighten the many bolts 8 again. Therefore, the replacement of the foil 3 is very laborious work and takes much time. For example, it is common to use three persons for 4 to 5 hours each to replace the foil in a conventional unit.
It is required that the force that couples the foil holder 2 to the flange 11 be uniformly applied. If the force is not uniform, the surfaces will not be parallel and a gap will be present at the edge of the foil 3 causing the tension or the foil to be locally increased. For that reason, the tightening torque for each of the bolts 8 must be controlled carefully to be a prescribed level so that the force that couples the foil holder 2 and the flange 11 to each other acts uniformly. It is laborious to control the tightening torque to the required degree on all the bolts 8. Moreover, since such work must be done by the persons in the irradiation chamber, the work can be dangerous. If a plurality of electron beam irradiators are installed in the same irradiation chamber, the operation of all of the irradiators must be stopped when the foil of one is being replaced because ozone or X-rays should be prevented from being generated. Even if the operation of all the electron beam irradiators is stopped, it is still dangerous to enter the irradiation chamber because the chamber will be full of ozone.